1. Field of the Invention
The present invention relates to a ball detection method and apparatus for wire-bonded points and more particularly to a method and apparatus for detecting balls after the ball bonding operations.
2. Prior Art
As shown in FIG. 4, in a workpiece 3 which comprises a semiconductor chip 2 installed on a lead frame 1, wires 4 are used for connecting the pads P.sub.1, P.sub.2 . . . of a semiconductor chip 2 and the leads L.sub.1, L.sub.2 . . . of a lead frame 1. The connection between the pads and the leads are done by a wire bonding apparatus as shown, for example, in FIG. 3.
Generally, in wire bonding executed between the pads and leads, any positional shift or discrepancy between them from predetermined positions is first detected at at least two points on the semiconductor chip 2 and at least two points on the lead frame 1 by a camera 11, and then the bonding coordinates for the pads and leads stored beforehand in the bonding apparatus are corrected based upon the detected result.
When the detection of the positional shift is performed by the camera 11, an X-axis motor 12 and Y-axis motor 13 are first driven so that the central axis 11a of the camera 11 is moved directly above each measuring point. After the bonding coordinates are corrected as described above, the capillary 15, through which a bonding wire 4 passes, is moved in the X and Y directions, or horizontally, and in the Z direction, or vertically, and the wire 4 is bonded between the pads P.sub.1, P.sub.2 . . . of the semiconductor chip 2 and the leads L.sub.1, L.sub.2 . . . of the lead frame 1.
In the process described above, the central axis 11a of the camera 11 and the central axis 15a of the capillary 15 are positionally offset by a distance W. Accordingly, after the positional shifts of points to be bonded have been detected by the camera 11 and the bonding coordinates have been corrected, an XY table or the bonding table 16 that has the capillary 15 is moved horizontally the offset distance W by the X-axis motor 12 and Y-axis motor 13 so as to bring the capillary 15 above a first bonding point. Then, the wire 4 is bonded at the point of corrected bonding coordinates by moving the XY table in the X, Y and Z directions by the X-, Y- and Z-axis motors by adjusting (or raising and lowering) the capillary arm 17 (or causing the capillary arm 17 to pivot).
In FIG. 4, the capillary arm 17 is pivotally mounted to a bonding head 10, and the camera 11 is fixed to the bonding head 10 via a camera-holding arm. In this Figure, Xw represents the X-axis component of the offset distance W, and Yw represents the Y-axis component of the offset distance W.
The devices disclosed in Japanese Patent Application Laid-Open (Kokai) Nos. 4-317342 and 4-320350 are prior art examples of wire bonding apparatuses of the type described above. The methods disclosed in Japanese Patent Application Laid-Open (Kokai) No. 51-78174 and Japanese Patent Application Publication (Kokoku) No. 57-50059 are prior art examples of methods for correcting the positions of workpieces.
As seen from the above, a mechanically determined fixed offset distance W exists between the central axis 11a of the camera 11 and the central axis 15a of the capillary 15. Accordingly, bonding can only be accurately performed at bonding points by detecting via the camera 11 the amount of positional shift of each workpiece 3, correcting the bonding coordinates, and then moving the capillary 15 (in accordance with a predetermined program) a distance that corresponds to the offset distance so that the capillary 15 is positioned at the corrected bonding coordinates.
However, wire bonding apparatuses generally include a heating block for heating lead frames, and they also include a capillary arm which holds the capillary. In addition to the heating block, other heat-generating sources such as X- and Y-axis motors and an ultrasonic oscillation source which is installed inside the capillary arm are used in wire bonding apparatuses. A Z-axis motor which raises and lowers or pivots a capillary arm is another element used in the bonding apparatuses.
Due to the operating heat and variations in the ambient temperature caused by the heat-generating sources as described above, differences between the thermal expansion of the capillary arm and the thermal expansion of the camera-holding arm that holds the camera are created. As a result, the offset distance between the central axis of the camera and the central axis of the capillary tends to change with respect to both amount and direction, and error caused by this fluctuation results in a shift in the bonding position.
The detection of the shift in the bonding position is generally accomplished by the imaging device 11 that detects the position of the center of the ball bonded to the pad P.
The method described in Japanese Patent Application Laid-Open (Kokai) No. 5-206197 is one of the conventional methods for detecting the position of the center of the ball. This method will be described with reference to FIGS. 5 through 7.
FIG. 5 shows that the center 7 of the pad P and the center 6 of the ball 5 bonded to the pad P substantially coincide, while FIGS. 6 and 7 illustrate a case where the center 5 of the ball 6 is shifted considerably from the center 7 of the pad P and not coincide.
In the method of this prior art, when ascertaining the approximate position of the ball 5, a macro-recognition process that uses two-value pattern matching is first performed to detect a tentative ball center 20. Using this tentative ball center 20 as a reference point, an edge search is next performed in eight directions 21a through 21h, so that multi-value edges 22a through 22h of the ball 5 are detected. Then, these detected ball edges are divided into four sets, i.e., 22a-22e, 22b-22f, 22c-22g and 22d-22h, and the respective center coordinates (intermediate positions) 23a, 23b, 23c and 23d are calculated or obtained. After this, perpendicular lines are drawn from the center coordinates 23a, 23b, 23c and 23d, and the center-of-gravity coordinates of the ball 5, i.e., the center coordinates 6 of the ball 5, are determined from the coordinates of the respective points of intersection.
As seen from the above, in the prior art described above, the tentative ball center 20 is set by subjecting the ball 5 to a two-value conversion, and edge searches are performed in eight different directions using this tentative ball center 20 as a reference point. As a result, several problems arise. The illuminating optical system is generally set so that the ball 5 and wire 4 are dark and pad P is bright; however, the area inside the ball 5 is not evenly dark but instead shows an irregular brightness. As a result, locations which are in fact no the edges of the ball are detected in error as ball edges.
In addition, in the prior art method described above, it is necessary to calculate the center coordinates 23a, 23b, 23c and 23d of all of the ball edge combinations 22a-22e, 22b-22f, 22c-22g and 22d-22h that pass through the tentative ball center 20. Accordingly, another problem arises. If the center 6 of the ball 5 and the center 7 of the pad P roughly coincide as shown in FIG. 5, the edges of the pad P and the edges of the ball 5 are separated from each other and can be clearly distinguished so that the center coordinates 23a, 23b and 23c are calculated. Thus, there is no problem. However, if the center 6 of the ball 5 is greatly shifted from the center 7 of the pad P so that the image of the ball 5 overlaps with the edges of the pad P or comes very close to the edges of the pad P as shown in FIGS. 6 and 7, the pattern surrounding the ball edges 22a and 22g are detected. This results in erroneous detections. Accordingly, in such cases, the center coordinates 23a of the edge combination 22a-22e and the center coordinates 23c of the edge combination 22c-22g are detected in error, and the result is that the center coordinates 6 of the ball 5 are not accurate.
Furthermore, in the prior art method, the ball edges 22a through 22h are first detected, the center points of the respective ball edge combinations are next determined, perpendicular lines are drawn from the center points, and then the center coordinates 6 of the ball are determined from the coordinates of the respective points of intersection. Thus, it takes a long time, and detection is not accomplished in a short period of time.
In recent years, furthermore, there has been a tendency toward a decreased pad size as a result of an increase in the number of wires, i.e., an increase in the number of pins. As a result, balls are formed so as to completely cover the pads. In such cases, erroneous detection of the ball edges may occur not only when the center of a ball is greatly shifted from the center of the pad but also even when the center of the ball substantially coincides with the center of the pad. Thus, as in the case described before, the center coordinates of the ball are not calculated accurately.